The present invention relates to cellular telephone systems, and more particularly to the adaptive assignment of channels to calls in analog cellular telephone systems as well as in digital systems having limited downlink interference measurement resources.
In cellular telephone communications systems (henceforth referred to as "cellular systems", or simply "systems"), it is important to maximize traffic-handling capacity, because the demand for such capacity continues to increase. One factor that affects capacity is the way the totality of available communication channels are allocated for use by particular cells within the system. The use of the same channel by two or more cells that are in relatively close proximity to one another can cause each cell to experience too much co-channel interference, and should therefore be avoided.
One solution for avoiding co-channel interference is to have each cell operate on a dedicated group of channels that are not used by any other cell in the system. Although this strategy successfully avoids the occurrence of co-channel interference, it limits the system's traffic-handling capacity to the number of channels that the system is permitted to use.
In order to increase the system's traffic-handling capacity, it is possible to devise a reuse plan whereby any one channel may be concurrently used by two or more cells. Co-channel interference is limited by ensuring that the one channel is allocated to cells that are geographically located far enough apart (referred to as the "reuse distance") so as not to severely interfere with one another. The appropriate distance for limiting interference will depend upon factors that affect the carrier to interference. ratio (C/I) on that particular channel in each cell.
A number of techniques have been devised for selecting and assigning traffic channels in a way that reduces the likelihood of co-channel interference in a cellular communication system using a fixed channel reuse plan, that is, a plan that does not change over time. The publication V. H. MacDonald, "Advanced Mobile Phone Service: The Cellular Concept", Bell System Technology Journal, pp. 15-41, January 1979, describes such a plan.
Fixed channel reuse plans are based upon assumptions about propagation conditions in order to guarantee minimum C/I in the system. However, such plans are very difficult and tedious to make, and these difficulties increase as the cells become smaller. Furthermore, the number of calls that are handled by any given cell may increase or decrease over time. Because of such evolving traffic patterns, not to mention the evolution of the system itself, cellular systems that utilize fixed channel reuse plans may suffer a degradation in traffic-handling capacity over time.
To avoid such degradation, an adaptive channel allocation (ACA) plan is preferable to a fixed channel reuse plan. In an ACA plan, as the name implies, the utilization of radio resources in the system adapts over time in order to accommodate changes in the current traffic and propagation situation. The adaptation is made on the basis of system measurements that are at least periodically made. The goal, in such a scheme, is to allocate channels so that all links have satisfactory quality. A common feature of ACA systems is that they allocate a channel out of a set of channels which fulfills some predetermined quality criteria. However, different ACA schemes utilize different criteria for selecting channels from the set.
The general concept underlying ACA systems is well-known to those having ordinary skill in the art. For example, H. Eriksson, "Capacity Improvement by Adaptive Channel Allocation", IEEE Global Telecomm. conf., pp. 1355-1359, Nov. 28-Dec. 1, 1988, illustrates the capacity gains associated with a cellular radio system where all of the channels are a common resource shared by all base stations. In the above-referenced report, the mobile measures the signal quality of the downlink, and channels are assigned on the basis of selecting the channel with the highest carrier to interference ratio (C/I level).
A different approach is described by G. Riva, "Performance Analysis of an Improved Dynamic Channel Allocation Scheme for Cellular Mobile Radio Systems", 42nd IEEE Veh. Tech. Conf., pp. 794-797, Denver, 1992, where the channel is selected based on achieving a quality close to or slightly better than a required C/I threshold. Also, Y. Furuya et al., "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communications Systems", Second Nordic Seminar on Digital Land Mobile Radio Communication, pp. 311-315, Stockholm, Oct. 14-16, 1986, describes an ACA system wherein the recent history of link quality is considered as a factor in allocation decisions. In addition, several hybrid systems have been presented where ACA is applied to a small block of frequencies on top of a fixed channel allocation scheme. Such an example is presented in K. Sallberg et al., "Hybrid Channel Assignment and Reuse Partitioning in a Cellular Mobile Telephone System", Proc. IEEE VTC '87, pp. 405-411, 1987.
Apart from increases in system capacity, adaptive channel allocation obviates the need for system planning. Planning is instead performed by the system itself. This feature of ACA is particularly attractive when system changes are implemented, when new base stations are added, or when the environment changes, for example by the construction or demolition of large buildings.
It is preferable to implement ACA schemes in two parts: a "slow" part, and a "fast" part. The "slow" part determines, for each cell, a set of channels to be used based on interference and traffic fluctuations that occur over a relatively long period of time (e.g., 20-30 busy hours, which could take several weeks to occur). This eliminates the frequency planning problem, and may also adapt to average traffic loads in the system. The "fast" part is concerned with selecting at any given moment, from the slowly determined set of channels, the "best" channel for each connection, based on short term interference measurements. Implementation of both the "slow" and the "fast" parts of an ACA scheme may be distributed in the system, so that each base station determines its portion of the frequency plan as well as channel assignments based on local observations within the cell.
One reason for splitting an ACA scheme into two parts (i e., "fast" and "slow") is because of the use of auto-tuned combiners that are mechanically tuned, by means of small motors, to desired frequency ranges. Tuning is an automatic, but slow, operation that cannot be performed when a call arrives at the cell.
Furthermore, each base station is equipped with a limited number of transceivers and is therefore not capable of using all channels simultaneously. By dividing the ACA scheme into "slow" and "fast" parts, a strategy can be developed wherein the combiners are tuned to a set of frequencies that are obtained from the "slow" ACA scheme, and then the "fast" part of the ACA scheme makes its channel selection from among the "slowly" determined set of frequencies. The Y. Furuya et al. publication, cited above, describes aspects of an ACA scheme which can be categorized as "slow" and "fast", as those terms are defined here
Having a slowly changing frequency plan provides an additional advantage in that it is easier to observe the interference in the system. This is important when considering the limited measurement resources in any given system.
For prior art ACA plans to work properly, it is essential that both downlink (i.e. from base station to mobile station) and uplink (i.e., from mobile station to base station) measurements of channel interference levels be made. For ACA to work at its best, accurate interference level measurements should be made on all channels.
Considering only digital cellular systems for the moment, ACA schemes are feasible because there are some resources for making both uplink and downlink measurements. Uplink measurements may be made by equipment in the base station. Downlink measurements may be made by a mobile station, which then reports its measured values back to the base station. However, it is still difficult to obtain measurements on all channels. For example, in digital systems such as D-AMPS, the mobile assisted handover (MAHO) facility is used to evaluate the downlink disturbance levels on traffic frequencies that are currently not in use in the serving cell. This MAHO measurement resource is very limited, however, because each mobile can only measure a few frequencies. As a result, it is not possible to get information about downlink interference on all frequencies in a cell within a short time frame, so the channel allocation has to be based, at least in part, on old information.
Considering now the task of devising an ACA scheme for use in analog systems, such as AMPS and TACS, one is faced with a difficult problem because analog systems typically do not have any provisions for making downlink measurements; the inability of mobile terminals to provide any information about the received downlink signal means that such systems are confined to measuring channel interference in the uplink direction only. As a result, channel allocation for analog systems has been manually planned in the prior art.
Because of the above-described benefits of utilizing ACA schemes for allocating cellular system resources, it is desirable to provide a technique that will allow such schemes to be applied in analog cellular systems as well as in digital cellular systems having very limited downlink interference measurement resources.